The most widely used means for processing thermoplastic materials into articles of desired shape is by injection moulding. A typical injection moulding cycle commences by locking mould halves together under a considerable clamping force. Hot, molten thermoplastic material is then injected rapidly into the mould cavity and pressure is maintained whilst the thermoplastic material cools and solidifies. Finally, when the material has cooled to a suitably rigid state, the mould halves are opened and the article of desired shape is ejected. The cycle then begins again to prepare the required number of shaped articles.
To be of value in injection moulding, the moulds need to be capable of withstanding repeated temperature and pressure shocks resulting from the injection of hot materials. The mould should withstand many cycles without melting, deforming or cracking.
An alternative method for producing three-dimensional articles called `stereolithography` is also known. This method comprises curing successive layers of a photopolymerisable liquid using a computer-controlled light source, e.g. a laser, to give the desired article. Stereolithography has been used to prepare prosthetic devices, medical models and a variety of other articles having complex internal structures. Hull's U.S. Pat. Nos. 4,575,330, 4,752,498 and Hideo Kodama's article in Rev. Sci. Instrum. 52 (11), 1770-1773, November 1981 describe this method in more detail.
A limitation of stereolithography is that it can only produce three dimensional articles from photocurable liquids. If one requires an article made from polythene, nylon or other meltable materials which are not derived from a photocurable liquid then stereolithography is of no direct use.
International patent application No.PCT/US89/03303 proposed a method for preparing a ceramic mould wherein a disposable pattern is produced by stereolithography, a ceramic material is formed around the pattern by applying a refactory material (e.g. an aqueous ceramic slurry) and the pattern is destroyed and removed by heating the mould. The patterns used in this application were therefore unstable to heat and were used only once.